ρ{\displaystyle \rho } is called the volumic mass. This law can be proved by calculations using elementary fluid volumes. So, variation of mass M{\displaystyle M} in volume dτ{\displaystyle d\tau } per time unit is opposed to the outgoing mass flow:

But this law can also be proved in calculating the first moment of the Vlasov equation (see equation eqvlasov). Volumic mass is then defined as the zeroth order moment of the repartition function times mass m{\displaystyle m} of one particle: